A/C and Heater not blowing after...

My A/C started doing this about a month ago.....now mt heat is doing it too.....it's fine for about 10 or 15 mins right when I start out.....then it's like it quits coming out....there would still be cold air or heat...and I can still hear the blower on...but it's like it doesn't come out....it's just kind of there.